In electrical and electronic circuits, electrically actuated switches or relays are commonly used for performing switching functions, i.e. opening and closing a circuit, in response to a control signal. In a relay-controlled circuit which is used for signaling or control functions, relatively small currents are switched. In many such applications the relay takes the form of a semiconductor or solid state switch. In other applications where relatively large currents are switched or because of the environment, electromechanical relays are commonly used. Solid state switching devices are now available for switching both AC and DC circuits at relatively large currents.
There are applications in which the operation of a relay is of vital importance in performing a control function and a failure thereof may result in equipment failure of serious consequence. For example, on control systems for safeguarding hazardous industrial machines, such as punch presses, a relay is employed to energize the press control circuit for starting and stopping the press. In an emergency situation, the press is brought to a stop by opening the relay switch to deenergize the control circuit. The relay is controlled by an intrusion detector on the press which produces a turn-off signal for the relay in case the operator reaches too close to the pinch point of the press. If the relay should fail during operation so that it is stuck in the closed state, the machine may cause injury to the operator or damage to the machine. In such applications, it is desirable to prevent relay failure from resulting in an unsafe circuit condition. It should be fail-safe and self-checking.
Electromechanical relays are in some respects well suited for control circuits of industrial machines. One favorable attribute is that the electromechanical relay exhibits a high degree of immunity to false switching due to voltage transients and other electrical noise which is often present in an industrial environment. One of the major drawbacks of the electromechanical relay is an inherently short life relative to that of solid state devices.
Solid state relays or switches for control circuits offer many advantages over electromechanical relays including faster switching, low power actuation, small size and long life. However, the solid state relay is inherently susceptible to electrical noise such that voltage transients with relatively low level rates of voltage change may produce unwanted switching of the relay. It is not suitable, without special associated circuitry, for use in industrial control circuits where false switching cannot be tolerated. If the solid state relay is rendered substantially immune to voltage transients, it is suited for many control circuit applications. However, for many applications it should be fail-safe and self-checking.
It is an objective of this invention to provide and improved electrically actuated switch or relay and self-checking circuits therefor.